News Excerpt:

Recent studies have raised concerns about the declining levels of dissolved oxygen (DO) in Earth's water bodies, posing a significant threat to aquatic life and ecosystems.

What is dissolved oxygen?

• Dissolved oxygen (DO) is the amount of oxygen that is present in water. 
• Water bodies receive oxygen from the atmosphere and from aquatic plants.

• Running water, such as that of a swift moving stream, dissolves more oxygen than the still water of a pond or lake.

What can dissolved oxygen tell us about the condition of water?

• DO is considered an important measure of water quality as it is a direct indicator of an aquatic resource’s ability to support aquatic life. 
• Levels of DO are measured with a calibrated water quality probe meter, usually in conjunction with measurements for temperature and pH. 
• While each organism has its own DO tolerance range, generally, DO levels less than 5 mg/L are considered stressful for fish and levels  less than 3 mg/L are too low to support fish. 
• DO levels below 1 mg/L are considered hypoxic and usually devoid of life.

What is Aquatic Deoxygenation?

• It refers to the reduction or loss of dissolved oxygen in water bodies, such as oceans, seas, rivers, lakes, and other freshwater and marine ecosystems.
• Low levels of oxygen (hypoxia) or no oxygen levels (anoxia) can occur when excess organic materials, such as large algal blooms, are decomposed by microorganisms which increases Biological Oxygen Demand (BOD).
• BOD represents the amount of oxygen consumed by bacteria and other microorganisms while they decompose organic matter under aerobic (oxygen is present) conditions at a specified temperature
• During the decomposition process, DO in the water is consumed. Low oxygen levels often occur in the bottom of the water column and affect organisms that live in the sediments. 
• In some water bodies, DO levels fluctuate periodically, seasonally and even as part of the natural daily ecology of the aquatic resource. 

Several factors contribute to this alarming trend:

• Warmer Waters: Rising global temperatures, driven by increasing greenhouse gas emissions, result in warmer water temperatures. Warmer waters are less capable of holding dissolved oxygen, leading to reduced levels in aquatic environments.
• Eutrophication : Excessive nutrient influx from sewage, industrial waste, and agricultural and domestic fertilizers promotes algal and bacterial growth. 
• These organisms consume large amounts of dissolved oxygen, further depleting its availability in the water.
• Organic Matter Accumulation: The increase in organic matter from various sources creates a higher BOD as it decomposes, exacerbating the depletion of DO.

Implications of Reduced DO

Impact on Aquatic Life: 

• Just as atmospheric oxygen is crucial for terrestrial animals, dissolved oxygen is vital for marine and freshwater organisms. 
• Reduced DO levels can lead to the suffocation and death of aquatic species, disrupting food chains and biodiversity.

Human Dependence on Aquatic Ecosystems: 

• Billions of people rely on freshwater and marine habitats for food and income.
• As per CBD, over three billion people depend on marine and coastal biodiversity for their livelihoods, and marine fisheries directly or indirectly employ over 200 million people and fish continues to be one of the most-traded food commodities worldwide.
• The decline in DO threatens these livelihoods by compromising the health and productivity of these ecosystems.

Planetary Boundaries and Ecological Integrity: 

• A team of scientists has proposed adding aquatic deoxygenation to the list of 'planetary boundaries'. 

• • These boundaries define thresholds within which humanity can develop sustainably. 

• Ignoring deoxygenation undermines the integrity of Earth's ecological and social systems and affects other planetary boundary processes.

Conclusion:

The depletion of dissolved oxygen in water bodies is a critical issue that requires immediate attention to protect aquatic ecosystems and the human populations dependent on them. Addressing this challenge involves mitigating climate change, controlling nutrient pollution, and preserving the health of our water resources.